Originally Posted by brad26
Does anyone know what actually happens, after fermentation has completed, inside the beer. I know flavors and aromas change, but I'm just curious as to WHY that happens. What chemically is actually going on in there?
Lots and lots of things. Most striking (and perhaps of most concern to brewers) are
1) Acetolactate which has leaked out into the beer is nonenzymatically oxidized to diacetyl which is then taken up by the yeast cells and reduced to acetoin and 2,3 butanediol which have much higher taste thresholds
2) Acetaldehyde is taken up by the yeast and reduced to ethanol.
There is a whole thread on these 2 reductions here but much of it is devoted to pedantic bickering over the details of how they take place. There is a useful tip on how to test for diacetyl in your beer, however (at the end).
Many other perhaps less dramatic flavor changes take place as well. Sulfides (jungbuket) get scrubbed out by CO2, other aldehydes get reduced, the CO2 bonds with the beer in some mysterious (to me anyway) manner, flavors mellow. I'm sure if you search the literature you'll find hundreds of papers on these as sending stable beer out from the brewery is a big concern for commercial operators.